Legendary or not, Pokémon are Pokémon, and they will always be. Though, some of these legendaries do have quite a lot of power in them, and compared to average ones, they're extremely hard to take down. These kind of Pokémon aren't eligible to battle against others aside from their own, so they have their own, separate tier known as Ubers. Not all legendaries are like this, though; some aren't as powerful, but have enough strength to go up against the likes of Pikachu, Scizor, Gyarados, Charizard, and every other that you see every day. Those, if you ask me, are all okay to be used on your team.